As shown in FIG. 1(A), the conventional insert-type drill has two square-shaped cutting inserts, which are disposed at inner and outer sides of the drill body. The inner and outer inserts are disposed opposite from each other with respect to a rotational axis of the drill body. FIGS. 1(B) and (C) show the inner insert and a cutting edge of the outer insert being rotated by 180 degrees with respect to the rotational axis of the drill body. Accordingly, they show a relative positional relationship between a cutting edge of the inner insert and a cutting edge of the outer insert during a drilling operation.
The diameter of the insert-type drill varies with the distance between the inner insert and the outer insert. When the inner insert and the outer insert are disposed as shown in FIG. 1(C), only the cutting edge of the inner insert protrudes at a region where the rotational loci of the cutting edges of the inner insert and the outer insert overlap. Thus, a workpiece is cut by only the inner insert during a drilling operation. However, when the inner insert and the outer insert are disposed more closely to each other as shown in FIG. 1(B), the cutting edges of the inner insert and the outer insert both protrude at the region where the rotational loci of the inner insert and the outer insert overlap. Thus, during a drilling operation, the workpiece is cut in turn by the two cutting edges. Accordingly, during a drilling operation, long and thin chips are generated to surround a side surface of the drill body not being parted and intervene between the drill body and the hole being drilled. This disturbs the drilling operation and further damages the workpiece. Such an occurrence is particularly harmful to the workpiece, which is made from a relatively soft material such as soft steel. Further, at the region where the rotational loci of the inner insert and the outer insert overlap, a cutting resistance force may be concentrated at the ends of the inner and outer inserts, which protrude in turn along the rotational axis of the drill body. Thus, the lifetime of the cutting insert is reduced and stable cutting performance may not be achieved.
Korean Laid-Open Patent Publication No. 2005-7569 discloses an insert-type drill, which is different from the above-mentioned drill. As an inner insert, the drill employs an insert shown in FIG. 2(A), which comprises first and second part edges 4 and 6 connected via a transition part edge 2, while employing a conventional square-shaped insert as an outer insert. FIG. 2(B) shows a positional relationship between the two cutting inserts. FIG. 2(C) shows a relative positional relationship between the cutting edges of the inner insert and the outer insert by rotating the inner insert shown in FIG. 2(B) by 180 degrees with respect to the rotational axis.
Korean Laid-Open Patent Publication No. 2005-7568 discloses another insert-type drill. The drill employs an insert shown in FIG. 3, which comprises first and second part edges 4′ and 6′ connected via a transition part edge 2′, as an inner insert, while employing an insert, which is mirror-symmetrical to the inner insert, as an outer insert.
In said drill, the cutting edges of the inner insert and the outer insert are generally parallel to each other at a region where the rotational loci of the inner insert and the outer insert overlap. Thus, even if the inner insert and the outer insert become closer, only the inner insert may be maintained to protrude in the overlapping region without allowing the outer insert to protrude in turn. Accordingly, as shown in FIG. 1(B), even if the inner insert and outer insert become closer, the generation of long and thin chips and the local concentration of cutting resistance, caused by the protrusion of the inner insert and outer insert in turn, will be prevented, compared to a structure that the cutting edges of an inner insert and an outer insert are disposed opposite from each other at a region where the rotational loci of the cutting edges of the inner insert and the outer insert overlap.
However, since the inner and outer inserts are not identical, the manufacture of each insert requires different metallic patterns of powder metallurgy. In addition, the production process should be individually controlled for each of the inserts. Further, a stock of the inserts should be separately managed into two types. When the inserts are mounted or substituted, great care must be taken in selecting and mounting a correct insert.